docs: def ref_cov_per_contig(nucmer_hits):
          '''Input is hits made by self._parse_nucmer_coords_file.
             Returns dictionary. key = contig name. Value = number of bases that
             match to the reference sequence.'''
          coords = AssemblyCompare.nucmer_hits_to_ref_coords(nucmer_hits)
          return {x: pyfastaq.intervals.length_sum_from_list(coords[x]) for x in coords}